The present invention relates to a method of measuring pressure by means of a pressure gauge having a resonant element, in particular for use in oil wells, and more particularly the invention relates to such a method in which an evacuated capsule containing a resonant element is placed in the fluid whose pressure is to be measured, a resonance characteristic of the element is measured, and the pressure is deduced from said characteristic.
Document U.S. Pat. No. 4,547,691 discloses sensors of this type, known as crystal quartz gauges (CQGs) in which the evacuated capsule and the vibrating beam it contains are both made of quartz. Such sensors have the advantages of being capable of operating in a hostile environment, at very high pressure (up to 1500 kg/cm2), and at high temperatures (up to 200xc2x0 C.), thereby making them particularly suitable for use in the field of oil exploration, particularly for continuous monitoring of deposits. Finally, they present very good accuracy, of the order of 0.01% of full scale.
The quartz beam is set into resonance by the piezoelectric effect and the frequency of its vibration is measured accurately. The pressure to which the cell is subjected is deduced therefrom. Since the frequency of vibration is relatively insensitive to the effects of aging, of drift, of fatigue due to stress relaxation, of thermoelectric effects, or of instability of the electronics, such quartz sensors are therefore also very stable and of good resolution, unlike membrane or strain-gauge pressure sensors.
Nevertheless, they have the drawback of being high in cost and of possessing dimensions that are relatively large.
The present invention seeks to mitigate those drawbacks.
More particularly, the invention seeks to provide a pressure gauge that is capable of operating under the same conditions as a quartz pressure gauge and of providing a measurement of the same accuracy, but which is much easier to industrialize and which is of much lower cost.
Still more particularly, the invention seeks to provide both a method of measuring pressure and a pressure gauge which can be used on a large scale in the oil industry, particularly for continuous monitoring of deposits, but which could also be used while drilling.
To this end, the invention firstly provides a method of measuring pressure by as pressure gauge having a resonant element, in particular for use in oil wells, in which an evacuated capsule containing a resonant element is placed in the fluid whose pressure is to be measured, a resonance characteristic of the element is measured, and the pressure is deduced from said characteristic, the method being characterized by the fact that a resonant element is used which is to be found, during measurement, in a stress state that is close to buckling.
It will be observed that the invention relates to sensors having a resonant element, i.e. sensors of the same type as the above-mentioned quartz sensors, and not membrane or strain-gauge sensors. Advantage is thus taken of the fact that the magnitude being measured is associated with a frequency of vibration and not with a deformation.
In addition, the sensitivity of the method of the invention is increased by the fact that the resonant element is in a stress state that is close to buckling. Under such circumstances, the stiffness of the beam tends towards zero. A very small variation in the compression exerted thereon therefore gives rise to a considerable variation in its frequency of vibration. This provides mechanical amplification of the sensitivity of the sensor.
Advantageously, the resonant element is made of silicon.
This makes it possible to benefit from the excellent mechanical properties of silicon, particularly when it is monocrystalline. In addition, technology associated with using monocrystalline silicon is thoroughly mastered and well adapted to mass production.
It will be observed that, other things being equal, the sensitivity of a resonant element sensor made of silicon would normally be much less than that of an equivalent quartz sensor. The method of the invention makes it possible to remedy that drawback because measurement is performed while the resonant element is in a stress state that is close to buckling.
Two implementations of the method can be envisaged.
Firstly, it is possible merely to excite the element to resonance and measure its frequency of vibration.
The sensor is then operating in an open loop mode. The stress state of the resonant element, and thus its frequency of vibration, depend on the pressure to be measured.
In the other implementation, the stress state of the resonant element is adjusted to the limiting condition for buckling within the range of pressures to be measured.
Its frequency thus remains constant, and pressure is measured by measuring the stress applied to the resonant element. The sensor is then operating in a servo-controlled mode.
In a particular implementation, it is the temperature of the resonant element that is adjusted, e.g. using the Joule effect, so as to keep it in a stress state close to buckling.
Under such circumstances, a thermal expansion stress is thus added to the mechanical stress exerted on the element by the pressure that is to be measured, with this being done in such a manner as to reach the desired proximity of the buckling state. The resonant element always vibrates at the same frequency and the electrical current used for adjusting its temperature is representative of the pressure to be measured.
The invention also provides, a pressure gauge having a resonant element, suitable for performing measurements in oil wells, the gauge comprising a resonant element of crystalline material placed in an evacuated capsule organized to be subjected to a pressure that is to be measured, means for setting said element into resonance, and means for deducing the pressure from a vibration characteristic of the element, the gauge being characterized by the fact that stress control means are provided to ensure that during measurement the resonant element is to be found in a stress state close to buckling, thereby amplifying the sensitivity of the sensor.
More particularly, the resonant element can be made out of silicon.
In a first embodiment, said vibration characteristic of the resonant element is its frequency of vibration.
In another embodiment, said vibration characteristic of the resonant element is an electrical magnitude to which said means need to be subjected in order to ensure that the element keeps a frequency of vibration that is constant in spite of pressure variations.
It will also be observed that in prior art quartz sensors, vibration is excited by piezoelectric means. Other means need to be envisaged for a sensor having a resonant element made of silicon, since silicon is not piezoelectric.
In the invention, the excitation means can be capacitive means or optical means.